The present invention relates generally to test apparatus, and more particularly to test apparatus for quickly and efficiently in-circuit testing capacitors.
Various types of test apparatus have been developed for measuring the capacitance of a capacitor. Typically, these apparatus have employed an AC bridge, wherein the capacitor under test is included in one leg of the bridge and compared against a known component in the opposite leg of the bridge.
Unfortunately, such bridge arrangements, because of the need for first reconnecting the capacitor and then balancing the bridge, are not satisfactory for performing the rapid in-circuit capacitor checking frequently performed by radio and television repairmen. The present invention is directed to a test apparatus specifically suited for this purpose.
Unlike prior-art test apparatus, the present invention functions by applying a low-level AC signal to a capacitor of a designated capacitance while the capacitor remains connected. The frequency of this signal is user-selected such that the reactance exhibited by the capacitor to the applied signal is a pre-determined known constant if the capacitor has the specified capacitance. If the capacitor is of either higher or lower capacitance, the reactance is not the predetermined constant and a meter or other indicator signals a deviation.
In particular, the reactance X.sub.c of a test capacitor of capacitance C is a function of the frequency f of an applied AC signal, according to the equation ##EQU1## If X.sub.c is a constant K.sub.x, then ##EQU2## and ##EQU3## where f is the frequency applied to the capacitor under test to achieve the predetermined reactance K.sub.x.
In the test apparatus of the invention, one such frequency f is selected in accordance with this equation for each designated capacitor capacitance C whereby the reactance constant K.sub.x is always achieved if the capacitor under test is of the designated capacitance.
The frequency selection is automatically accomplished by the user simply selecting the designated capacitance allowing for a quick check of the capacitor without the user having to refer to charts or balance a bridge.